1. Field of the Invention
The invention relates to a laser interference lithography (LIL) apparatus with one or more optical fibers.
2. Related Art
LIL has been a low-cost and well-established method to produce nano-scale surface patterns that have high resolutions. LIL uses coherent light beams, which are split and reflected so that two or more than two beams overlap in the space, and interference fringes and periodic structures are formed on a photosensitive layer such as photoresist. LIL has the following advantages. First, LIL does not require a photomask or an imaging projection lens set with high numerical aperture (NA) value, wherein the mask and the lens set are used in the conventional projection photolithography technology. Second, a simple single transverse mode laser can be used instead of a highly incoherent transverse mode laser requiring beam homogenizing technology as required in projection photolithography. Third, plane or spherical wave sources can be used such as those produced by simpler single transverse mode lasers, instead of broad angle of incidence photomask illumination with specified sizes and shapes of partial coherence. Thus, the optical diffraction and the expensive apparatus can be avoided. Fourth, LIL has a large depth of focus, and can significantly decrease the influence of the environment on the exposure. Fifth, the incident angle of interference can be modified to obtain the smaller period and line width without modifying the optical device and the optical design.
Using LIL, nano-scale patterns can be produced through direct use of the exposed photoresist, or the pattern can be transferred to other structures and materials using a variety of material depositing and etching techniques. These structures can further be used in processes including nanoimprint lithography to reproduce the structures further.
In many applications of these nanostructures, including high density magnetic recording media and photonic devices, their performance is highly dependent on feature details such as period and diameter or line width. Therefore, a variety of these structures must be explored experimentally. Therefore, a system capable of manufacturing the uniform nano-structure, quickly changing the period and the line width or diameter of the nano-structure, is extremely valuable to accelerate the progress in process and device development.
However, LIL needs the very high laser pointing stability. Under the nanometer scale, either the laser pointing drifting or the vibration causes the change of the period or position of the interference pattern, resulting in the loss in contrast ratio or pattern shape. In extreme cases no interference fringes can be recorded in the photoresist. Although the real-time correction can be performed using the precise measurement and control, the burden of the relatively high costs and complexity of the apparatus and the technique are a substantial disadvantage.
In addition, all laser sources will produce spatial noise to varying degrees. To prevent wasted heat generated by the laser from affecting the environment of the interference lithography, the laser source is usually disposed on an optical table away from the interference lithography. This requires additional optical elements in the laser beam path. Imperfection in these devices, including lenses, mirrors, and contaminants on their surfaces can produce further spatial noise. A short focal length lens is used to disperse the noise with the higher spatial frequency away from the optical axis of the laser beam during the light transmission process, and a pin hole is disposed to serve as a spatial filter for filtering out the spatial noise of the laser beam or filtering out the diffracted light beam caused by the defect of the optical device on the optical path.
However, the position of the pin hole has to be properly adjusted because the poor effect is caused when the pin hole is too close to or away from the laser source, or drifts laterally, perpendicular to the beam direction. Thus, a fringe-locking system is often required using a complicated optical mechanism, an optical detection device and a precise optical device in conjunction with the high-speed and real-time feedback system, and this establishing process is time consuming and adds substantially to the cost and complexity of construction and operation.